Remotely powered electronic devices and related systems are known. For example, U.S. Pat. No. 5,099,227 issued to Geiszler et al. entitled Proximity Detecting Apparatus, discloses a remotely powered device which uses electromagnetic coupling to derive power from a remote source and then uses both electromagnetic and electrostatic coupling to transmit stored data to a receiver often collocated with the remote source. Such remotely powered communication devices are commonly known as radio frequency identification ("RFID") tags.
Radio frequency identification tags and associated systems have numerous uses. For example, radio frequency identification tags are frequently used for personal identification in automated gate sentry applications protecting secured buildings or areas. These tags often take the form of access control cards. Information stored on the radio frequency identification tag identifies the person seeking access to the secured building or area. Older automated gate sentry applications require the person accessing the building to insert or swipe their identification tag or card into or through a reader for the system to read the information from the identification tag. Newer radio frequency identification tag systems allow the radio frequency identification tag to be read at a small distance using radio frequency data transmission technology, thereby eliminating the need to insert or swipe an identification tag into or through a reader. Most typically, the user simply holds or places the radio frequency identification tag near a base station, which is coupled to a security system securing the building or area. The base station transmits an excitation signal to the radio frequency identification tag that powers circuitry contained on the radio frequency identification tag. The circuitry, in response to the excitation signal, communicates stored information from the radio frequency tag to the base station, which receives and decodes the information. The information read is used by the security system to determine if access is appropriate. Also, radio frequency identification tags are written remotely by an excitation signal appropriately modulated in a predetermined manner.
In addition to typical applications for access control of persons, RFID tags are useful in applications identifying things, such as electronic animal identification, baggage tracking, parcel tracking, inventory management applications, asset identification and tracking, and other applications. These applications involve transmitting stored information from a tag to an exciter/reader system in close proximity with the tag. These applications may also involve transmitting information from the exciter to the tag for storage on the tag.
Earlier RFID tags and systems primarily use electromagnetic coupling to remotely power the remote device and couple the remote device with an exciter system and a receiver system. The exciter generates an electromagnetic excitation signal to power up the device and the receiver receives the signal produced by the remote device.
Earlier electromagnetic coupling mechanisms include an oscillator as part of the exciter circuitry and a coil antenna on both the exciter circuitry and the remote tag that includes an electronic circuit. For example, in an earlier system, excitation circuitry is connected to a coil antenna, which radiates excitation signals that are picked up by a coil antenna mounted on a tag that also contains the electronic circuit. The excitation signals energize the circuit, which then provides an information-carrying signal that is transmitted to the receiver using electromagnetic or electrostatic coupling.
One problem with the use of electromagnetic coupling between a remote device and either an exciter or a receiver has been the complexity involved in the manufacture of remote devices that employ a coil antenna. The spiral layout of a typical coil antenna makes it more difficult to produce, increases cost and also the size of the remote device. Also, the coil antennas require tight tolerances for efficient performance.
Electrostatic coupling for RFID tags has been proposed to address the problems and cost associated with electromagnetic coupling. For electrostatically coupled RFID tags, the antenna coil or antenna coil and capacitor external to the electronic circuit are eliminated and replaced with cheaper electrostatic antennas which may be as simple as conductive ink printed on paper. The virtues of electrostatically coupled RFID tags are discussed in commonly assigned pending U.S. patent application Ser. No. 08/540,813, filed Oct. 11, 1995 by Ted Geiszler et al., titled "Remotely Powered Electronic Tag and Associated Exciter/Reader and Related Method", now abandoned; U.S. patent application Ser. No. 09/031,848 filed Feb. 27, 1998 by Victor Allen Vega et al., titled "Radio Frequency Identification Tag System Using Tags Arranged For Coupling to Ground"; U.S. patent application Ser. No. 09/041,480, filed Mar. 12, 1998 by Victor Allen Vega et al., titled "Radio Frequency Identification Tag Arranged For Magnetically Storing Tag State Information", now U.S. Pat. No. 6,040,773; and U.S. patent application Ser. No. 09/045,357, filed Mar. 20, 1998 by Victor Allen Vega et al., titled "Radio Frequency Identification Tag With a Programmable Circuit State," the disclosures of which applications are hereby incorporated by reference.
Electrostatic RFID tags typically use the same functional circuit blocks that are adapted for use with electromagnetic RFID tags. However, the characteristics of electrostatic coupling require different circuit considerations from Electromagnetic coupling. One characteristic of electrostatic coupling that is different from electromagnetic coupling is the nature of the electrostatic and electromagnetic fields, and in particular, the roll-off in coupling voltage associated with the fields. This characteristic affects read ranges. Also, the impedance characteristics for electrostatic coupling are different from the impedance characteristics for electromagnetic coupling. Although some circuits for electromagnetic coupling have proved somewhat suitable for electrostatic coupling, circuit techniques exploiting the unique characteristics of electrostatic coupling have not been employed.
Therefore, there is a need for electronic circuits and methods of creating the same, which are optimized for the characteristics of electrostatic coupling of RFID tags.